Re: 4941bis and multicast & QOS

[Mark Smith <[e-mail]>]

On Fri, 14 Feb 2020 at 12:44, Gyan Mishra <[e-mail]> wrote:
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> On Thu, Feb 13, 2020 at 8:21 PM Mark Smith <[e-mail]> wrote:
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>> On Fri, 14 Feb 2020, 11:53 Gyan Mishra, <[e-mail]> wrote:
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>>> Sorry Bert I missed your reply - responding now
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>>> On Sun, Feb 2, 2020 at 3:36 PM Manfredi (US), Albert E <[e-mail]> wrote:
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>>>> From: ipv6 <[e-mail]> On Behalf Of Gyan Mishra
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>>>> > How would multicast ASM or SSM work with the privacy extension temporary address enabled.  With the privacy temporary address enabled the IGMP Join would go out the temporary addresses, just as would normal unicast traffic.
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>>>> That's a good question, although only SSM would be a problem. In ASM, the IGMP/MLD join reports always go to the group multicast address anyway, and presumably, that won’t change. The unique source address of the client joining, or of the multicast source, should not matter. When responding to a new IGMP query, a new source address could be used in the report, still indicating to the router that membership exists. I would think, no problem.
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>>>    You make a good point as to where the problem may fall.  So with ASM model the receiver is not source aware and so sends a IGMP join via temporary address RPF interface for outgoing “client like” unicast connections.  The LHR DR on the subnet creates MRIB state and forwards *,G shared tree join along RPF path to the Anycast RP /MSDP - which has active SA cache entry for the Source within the multicast routing domain.  The LHR now learns the SA for the group and initiates SPT switchover to the SPT tree and builds a tree directly to the source.
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>>> With SSM the receiver is source aware via MLDv2, which now has source field in the packet for the SSM channel - along with well know default SSM group 232/8 - receiver is able to send via temporary address S,G join and to its LHR DR router on the subnet - which now builds SPT tree directly to the source.
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>>> So this all works well in a client / server multicast model with pim sparse-mode where the clients are receiver only and the server is source only.
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>>> In a peer2peer multicast model where the source acts as both receiver and source and the receiver acts as both receiver and source is the “bi-dir PIM” model -  this is the MPLS MVPN world translates into MP2MP LMDT instantiated via mLDP.
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>>> Multicast is predominately used in enterprises.  The internet MBONE never really took off due to multicast use of UDP RTP  transport for streaming and thus a requirement for end to end QOS for reliability.
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>>> So in the enterprise bi-dir pim model application requiring peer2peer multicast is the only scenario where you would run into issues with the privacy temporary address changing and becoming invalid resulting in session reset.  The app based source discovery would have to trigger changing of SSM channel when the source becomes invalid so the session would not reset.
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>>> I think for this peer2peer model to work you really need a stable address on the host.
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>> No you don't.
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>> You need an address that is stable enough for the length of the communication session, with stable enough meaning having a lifetime long enough that it doesn't disrupt the communication session.
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>> How long are you multicast sessions going to be?
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>  Since this is Clint to Client P2P no real servers involved now I think we would be fine even with 4941bis PL 1 VL 2.
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I think you still need to think more about it. I wouldn't agree that
things would be fine with PL 1, VL 2.

How long are your multicast sessions (or more generally, your
application communication sessions) going to be? What is the
requirement or requirements for address stability and persistence?

If you don't know answers to those questions, you can't determine when
temporary addresses with the RFC4291bis default lifetimes wouldn't be
adequate, and stable addresses should be used instead.


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>> If the length of the communication session is going to be less than the valid lifetime of a temporary address - by default 1 week - then there is no issue at all with using a temporary address, and in the interests of privacy it would be better to use a temporary address over an RFC7217 stable address.
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>> A server could have only temporary addresses as long as (a) communications sessions with clients are shorter than 1 week, and (b) the server updates DNS with new temporary address AAAA records dynamically.
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>> Stable addresses for servers is mainly just about avoiding dynamically updating DNS.
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>> Note that RFC7217 or RFC4862 EUI-64 addresses aren't permanent either. They too have lifetimes, PL=7 days, VL=4 weeks.
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>> They will likely persist longer than that, however that is totally dependent on them receiving RAs or DHCPv6 refreshing and extending the lifetime, and the interface staying attached to the same link.
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>    The router is always sending periodic RA so the lifetime is continually being reset.  So in essence the address is stable and in fact permanent

The word Permanent means never ever goes way. These addresses are not
permanent - they will go away if the conditions I've described aren't
met. They are persistent and stable, but not permanent.

>as long as the device remains attached.  If the device is attached and permanent  non mobile device the address would be permanent due to the periodic RAs from default gateway.  Cisco and other vendor implementations have the option to set the valid lifetime on the RA to maximum value which is a 32 bit number and that in fact sets it to infinite.

This is the IETF working group that specified that 32 bit number and
defined that 0xffffffff means 'infinite' in RFC4861. No need to state
it the way you have, it is assumed knowledge here.


> The knob is there however since the RA is periodic constantly resetting the VL, not sure why you would ever hardcore the VL in the RA to infinity.
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Because you don't want addresses to be permanent, because you want to
facilitate phased renumbering. This is why the default PL and VL in
RFC 4861 are 1 week and 4 weeks, not infinite.

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>> RFC7217/RFC4862 stable addresses aren't permanent addresses. Permanent addresses would have infinite lifetimes and never expire (the loopback address is a permanent one, as are link-local addresses, because they have infinite PL and VL).
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>     See above
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>>> This could be one use case where RFC 4941bis would fail - with removing the stable address as it’s currently written.
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>>>> In SSM, the IP destination address of the reports is a well-known multicast address, to routers, but of course, if you want to include only a certain set of sources, those source IP address(es) in the IGMP/MLD report either have to be valid, or you get nothing.
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>>>> > The temporary address would be the  RPF path shared and SPT tree.
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>>>> Isn’t this just related to multicast routing, between routers? Their addresses would not change at a dizzying rate?
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>>>> Bert
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>>> --
>>>
>>> Gyan  Mishra
>>>
>>> Network Engineering & Technology
>>>
>>> Verizon
>>>
>>> Silver Spring, MD 20904
>>>
>>> Phone: [phone]
>>>
>>> Email: [e-mail]
>>>
>>>
>>>
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>
> --
>
> Gyan  Mishra
>
> Network Engineering & Technology
>
> Verizon
>
> Silver Spring, MD 20904
>
> Phone: [phone]
>
> Email: [e-mail]
>
>
>